Magnetic Random Access Memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. One type of MRAM cell is a spin torque transfer MRAM (STT-MRAM) cell, which includes a magnetic cell core supported by a substrate. The magnetic cell core includes at least two magnetic regions, for example, a “fixed region” and a “free region,” with a non-magnetic region in between. The free region and the fixed region may exhibit magnetic orientations that are either horizontally oriented (“in-plane”) or perpendicularly oriented (“out-of-plane”) relative to the width of the regions. The fixed region includes a magnetic material that has a substantially fixed magnetic orientation (e.g., a non-switchable magnetic orientation during normal operation). The free region, on the other hand, includes a magnetic material that has a magnetic orientation that may be switched, during operation of the cell, between a “parallel” configuration and an “anti-parallel” configuration. In the parallel configuration, the magnetic orientations of the fixed region and the free region are directed in the same direction (e.g., north and north, east and east, south and south, or west and west, respectively). In the “anti-parallel” configuration, the magnetic orientations of the fixed region and the free region are directed in opposite directions (e.g., north and south, east and west, south and north, or west and east, respectively). In the parallel configuration, the STT-MRAM cell exhibits a lower electrical resistance across the magnetoresistive elements (e.g., the fixed region and free region). This state of low electrical resistance may be defined as a “0” logic state of the STT-MRAM cell. In the anti-parallel configuration, the STT-MRAM cell exhibits a higher electrical resistance across the magnetoresistive elements. This state of high electrical resistance may be defined as a “1” logic state of the STT-MRAM cell.
Switching of the magnetic orientation of the free region may be accomplished by passing a programming current through the magnetic cell core and the fixed and free regions therein. The fixed region polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the core. The spin-polarized electron current exerts torque on the free region. When the torque of the spin-polarized electron current passing through the core is greater than a critical switching current density (Jc) of the free region, the direction of the magnetic orientation of the free region is switched. Thus, the programming current can be used to alter the electrical resistance across the magnetic regions. The resulting high or low electrical resistance states across the magnetoresistive elements enable the read and write operations of the STT-MRAM cell. After switching the magnetic orientation of the free region to achieve the parallel configuration or the anti-parallel configuration associated with a desired logic state, the magnetic orientation of the free region is usually desired to be maintained, during a “storage” stage, until the STT-MRAM cell is to be rewritten to a different configuration (i.e., to a different logic state).
Switching of the magnetic orientation of the free region of a magnetic memory cell including a magnetic tunnel junction (“MTJ”) may be affected by the tunnel magnetoresistance (“TMR”). The TMR of a MTJ is a function of the resistance between an upper electrode and a lower electrode between which the MTJ is disposed, in the high electrical resistance state and the low electrical resistance state. Specifically, the TMR measures the difference between a cell's electrical resistance in the anti-parallel configuration (Rap) and its electrical resistance in the parallel configuration (Rp) to Rp (i.e., TMR=100·(Rap−Rp)/Rp). Thus, the TMR is equivalent to the change in resistance observed by changing the magnetic state of the free layer. Generally, a MTJ with a homogeneous crystal structure (e.g., a bcc (001) crystal structure), having few structural defects in the microstructure of its magnetic material, has a higher TMR than a MTJ with structural defects. A magnetic memory cell with high TMR may have a high read-out signal, which may speed the reading of the cell during operation. A higher TMR is preferred for a reliable read operation as it will generate a larger signal difference between the on and off states of the cell. In other words, the higher the TMR, the more sensitive the device, and the easier to distinguish between logic states of an associated magnetic memory cell.
Patterning of magnetic memory cells often includes reactive ion etching (RIE), which may introduce chemical damage to such memory cells. Reactive ion etching may include exposing the magnetic memory cell to one or more gases including halogen-based ions, hydrogen ions, oxygen ions, or other reactive gas components that may undesirably react with the magnetic materials and a tunnel barrier material of the magnetic memory cells. Undesired reactions between the magnetic materials or the tunnel barrier material and reactive gases may affect the crystal structure of the magnetic material cell and undesirably alter properties of the magnetic memory cell.
Ion beam etching (IBE) is a potential alternative for patterning of MRAM cells. However, conventional ion beam etching may damage the relatively thin materials of the magnetic materials of the memory cells. At conventional ion beam etching energies (e.g., about 200 eV), noble gases and other ion sources included in the ion beam may implant into the magnetic material, often several monolayers into an exposed surface of the magnetic material. The implanted materials may distort a crystal structure and induce lattice distortion in the magnetic materials, which may negatively alter the magnetic properties of the magnetic materials and associated memory cells. For example, intermixing and diffusion of elements of the ion beam and of adjacent materials in the magnetic stack may reduce one or more of a coercitivy (Hc), a magnetism, or a tunnel magnetoresistance (TMR), and may increase a resistance (i.e., an increased switching current, Jc), of the magnetic memory cell.
In addition, materials of the magnetic cell structure may exhibit a relatively low vapor pressure and may not, therefore, be carried out of an etch chamber during or after patterning. Accordingly, such materials may resputter (e.g., redeposit) on sidewalls of memory cell stack structures being patterned during the etching process. The resputtered material may be electrically conductive and cause an electrical short between adjacent memory cells and between, for example, upper and lower electrodes of the same memory cell.